Highway departments use a variety of techniques to monitor traffic in an effort to detect, mitigate, and prevent congestion. Typically, each highway department has a command center that receives and integrates a plurality of signals which are transmitted by monitoring systems located along the highway. Although different kinds of monitoring systems are used, the most prevalent system employs a roadway metal detector. In such system, a wire loop is embedded in the roadway and its terminals are connected to detection circuitry that measures the inductance changes in the wire loop. Because the inductance in the wire loop is perturbed by a motor vehicle (comprising a quantity of ferromagnetic material) passing over it, the detection circuitry can detect when a motor vehicle is over the wire loop. Based on this perturbation, the detection circuitry creates a binary signal, called a "loop relay signal," which is transmitted to the command center of the highway department. The command center gathers the respective loop relay signals and from there makes a determination as to the likelihood of congestion. The use of wire loops is, however, disadvantageous for several reasons.
First, a wire loop system will not detect a motor vehicle unless the motor vehicle comprises a sufficient ferromagnetic material to create a noticeable perturbation in the inductance in the wire loop. Because the trend is to fabricate motor vehicles with non-ferromagnetic alloys, plastics and composite materials, wire loop systems will increasingly fail to detect the presence of motor vehicles. It is already well known that wire loops often overlook small vehicles. Another disadvantage of wire loop systems is that they are expensive to install and maintain. Installation and repair require that a lane be closed, that the roadway be cut and that the cut be sealed. Often too, harsh weather can preclude this operation for several months.
Other non-invasive systems have been suggested. U.S. Pat. No. 5,060,206, patented Oct. 22, 1991, by F. C. de Metz, Sr., entitled "Marine Acoustic Aerobuoy and Method of Operation," provided a marine acoustic detector for use in identifying a characteristic airborne sound pressure field generated by a propeller-driven aircraft. The detector included a surface-buoyed resonator chamber which was tuned to the narrow frequency band of the airborne sound pressure field and which had a dimensioned opening formed into a first endplate of the chamber for admitting the airborne sound pressure field. Mounted within the resonator chamber was a transducer circuit comprising a microphone and a preamplifier. The microphone functioned to detect the resonating sound pressure field within the chamber and to convert the resonating sound waves into an electrical signal. The preamplifier functioned to amplify the electrical signal for transmission via a cable to an underwater or surface marine vehicle to undergo signal processing. The sound amplification properties of the resonator air chamber were exploited in the passive detection of propeller-driven aircraft at airborne ranges exceeding those ranges of visual or sonar detection to provide 44 dB of received sound amplification at common aircraft frequencies below 100 Hz. However, this patent used only a single electro-acoustic transducer for receiving acoustic signals within a detection zone, and did not teach spatial discrimination circuitry for representing acoustic energy emanating from a detection zone.
U.S. Pat. No. 3,445,637, patented May 20, 1969, by J. M. Auer, Jr., entitled "Apparatus For Measuring Traffic Density" provided apparatus for measuring traffic density in which a sonic detector produced a discrete signal which was inversely proportional only to vehicle speed for each passing vehicle. A meter, which was responsive to the discrete signals, produced a measurement representative of traffic density. However, this patent used only a single electro-acoustic transducer for receiving acoustic signals within a detection zone, and did not teach spatial discrimination circuitry for representing acoustic energy emanating from a detection zone.
U.S. Pat. No. 3,047,838, patented Jul. 31, 1962, by G. D. Hendricks, entitled "Traffic Cycle Length Selector" provided a traffic cycle length selector which automatically related the duration of a traffic signal cycle to the volume of traffic in the direction of heavier traffic along a throughfare. The Hendricks system did not teach the use of electro-acoustic transducers, but instead used pressure-sensitive detectors. While Hendricks employed plural, non-electro-acoustic transducers, the traffic cycle length selector system did not include spatial discrimination circuitry. Hendricks merely described the use of the output of several spatially discriminate detectors to generate a spatially indiscriminate signal.